Attachment 8 - Interim Stormwater Management Plan - TOPO · 132005.638.06

R3341: NOOSA LANDFILL: RRA CONCEPT HES BASIN AND DRAINAGE DESIGN REPORT



1                INTRODUCTION
This concept design report applies to the proposed construction of the RRA High-Efficiency Sediment (HES) Basin located
near the SW2 discharge point. This design report has been provided to detail catchments, and conceptual basin and
drainage sizing and locations.

2                CATCHMENT TO BASIN
The maximum catchment assumed to drain to the sediment basin is 7.83 hectares.

Details on the contributing catchment to the basin is provided in Appendix A.

3                DRAINAGE DESIGN
3.1.                DIVERSION DRAIN TO BASIN

The diversion drain to the basin has been sized for the 1 in 10 year ARI critical storm event in accordance with the
Guideline - Stormwater and environmentally relevant activities (DES, 2019). The Environmental Authority (EPPR01855314)
also specifies that the design event for temporary drainage works to convey waters to the sediment basin and around
disturbance areas is a 1 in 10 year ARI event plus 150mm freeboard for > 24 months disturbance.
Conceptual drain sizing is provided in Appendix A.
In addition to the 1 in 10 year ARI (10% AEP) drain sizing, conceptual drain sizing for a 100 year ARI (1% AEP) has also
been provided.

4                SEDIMENT BASIN SIZING AND DETAILS
4.1.                SEDIMENT BASIN SIZE

The Environmental Authority (EPPR01855314) specifies that site erosion protection measures and sediment control
measure must be implemented to have the hydraulic effectiveness to treat stormwater for 90% of the annual runoff for
each stormwater catchment.

A design report (Ref: R1767) and HES basin assessment was undertaken in conjunction with the SW5 HES basin design.
The assessment determined a 1 in 1 year ARI Type A basin will exceed the performance of a 10 year 24 hour traditional
sediment basin and also achieve the 90% requirement of annual runoff treatment specified in the Environmental
Authority.

The RRA HES basin was sized in accordance with this assessment with calculations provided in Appendix B with a
Concept design provided in Appendix C.

Noosa Shire Council also requested sediment basin sizes be provided for a 20 year, 50 year and 100 year ARI. The basin
size provided is for the 1 year ARI as detailed above, with the indicative increase in volume for the other storm events
provided below. It should be noted that water quality treatment for a 100 year ARI storm event is very extreme with such
events exceeding drain capacities and also likely to have site wide flooding impacts from local and regional drainage
paths.

       +      1 year ARI ~ 2,900m3 Settling Zone
       +      20 year ARI ~ 19,000m3 Settling Zone
       +      50 year ARI ~ 28,000m3 Settling Zone
       +      100 year ARI ~ 36,000m3 Settling Zone

4.2.                SEDIMENT BASIN OUTLET

The outlet of the sediment basin has been determined to be:

       +      Piped Outlet from Decant (RL 11.80 USIL, RL 11.56 DSIL)
       +      Emergency Spillway (RL 12.80)

The outlet levels were determined in conjunction with NSC to ensure the piped outlet would be above the downstream
levels and the Emergency spillway was above the 100 year ARI flood event of RL 12.64 (Noosa Landfill RRA Flood
Assessment, Hydrology & Water 2022). Due to the piped outlet being below the 1 year to 100 year ARI flood levels, a

Environment. Engineering. Education.                                                                             PAGE 1